In recent years, there has been a need for an optical network system that is able to accommodate various applications of different communication requirements. Thus, a communication technology has been discussed that multiplexes optical signals transmitted and received using different communication schemes. For example, with respect to an application that transmits large volumes of data, a higher-order modulation scheme is used. In this case, a generation and a demodulation of a modulated optical signal are performed using a digital signal processing technology. Note that the “higher-order” indicates that the number of bits per symbol is larger, for example, two or more. On the other hand, with respect to an application that requires a low-latency data transmission (or a data transmission of a small transmission delay), a transmission technology is used that minimizes the number of “optical-electric-optical” conversions in a communication route (such as intensity modulation/direct detection (IM/DD)).
As a related technology, an optical communication system is proposed that is able to add a transmission system of a differently-modulated signal to a single optical fiber network (for example, Japanese Laid-open Patent Publication No. 2011-244436). Further, an optical multiplexer is proposed that multiplexes pieces of information densely and transmits densely-multiplexed information (for example, Japanese Laid-open Patent Publication No. 2013-51541). Furthermore, Documents 1 and 2 below respectively disclose an optical network that performs a communication in a data center and a communication between data centers.    Document 1: Payman Samadi et al., Virtual Machine Migration over Optical Circuit Switching Network in a Converged Inter/Intra Data Center Architecture, OSA Optical Fiber Communication Conference 2015, Th4G.6    Document 2: Payman Samadi et al., Experimental Demonstration of Converged Inter/Intra Data Center Network Architecture, In 17th International Conference on Transparent Optical Networks, IEEE, We.B.3.3
As described above, when data volume is large, an optical signal is generated by a higher-order modulation scheme in order to transmit data efficiently. In general, a transmittable distance is short when a data transmission is performed by a higher-order modulation scheme, so a large number of relay devices are required between a transmitter and a receiver. Further, in many cases, the data transmission performed by a higher-order modulation scheme is realized using digital signal processing whose processing delay is large. Thus, it is difficult to lower a latency of a data transmission performed by a higher-order modulation scheme. Specifically, a latency becomes larger in the configuration in which an intensity modulated optical signal is converted into an electric signal and a modulated optical signal is generated from the electric signal in a higher-order modulation scheme using digital signal processing.
For example, an intensity modulation/direct detection (IM/DD) is used in order to realize a low-latency data transmission. In this case, a latency is low because an optical signal generated by intensity modulation is transmitted up to a destination without any conversion. However, the number of bits transmitted in one symbol is small, which results in reducing the communication resource utilization efficiency.